Method for identifying the position of a portable transponder, and an antitheft system

Abstract

A method for the recognition of the proximity of a portable transponder ( 20 ) to a base station ( 2 ), comprises the steps of: the base station transmits a first test signal with a defined power, the transponder determines the field strength with which it received the first test signal, and transmits a response signal to the base station containing said field strength and the base station stores the same. The transponder transmits a second test signal with a defined power, the base station determines the field strength with which it receives the second test signal and determines, from a comparison of the two field strengths, whether the transponder is in the vicinity thereof.

1. A method for identifying whether a portable transponder is located in the near area of a transmitting antenna of a base station, said method comprising:
 the base station sends a first test signal with a defined power level, the transponder determines the field strength with which it receives the first test signal and sends a response signal to the base station, said response signal contains the field strength of the first test signal, the base station stores the field strength with which the transponder received the first test signal, the transponder sends a second test signal with a defined power level, the base station determines the field strength with which it receives the second test signal, and the two field strengths are compared in order to determine whether the transponder is located in the near area of the transmitting antenna of the base station. 
 
   
   
     2. The method as claimed in  claim 1 , in which the information contained in the response signal relating to the field strength of the first test signal is encrypted. 
   
   
     3. The method as claimed in  claim 1 , in which the transponder sends the response signal essentially at the same time as the second test signal. 
   
   
     4. The method as claimed in  claim 2 , in which the transponder sends the response signal essentially at the same time as the second test signal. 
   
   
     5. An antitheft system for a motor vehicle, said system comprising:
 a base station fixed to the vehicle and having a transmitting/receiving unit for wireless communication with a transmitting/receiving unit of a portable transponder having code information stored therein, said code information may be sent from the transponder to the base station for authorization, wherein specific vehicle functions are being enabled only after a positive authorization check, the base station and the transponder are designed such that the base station sends a first test signal with a predetermined power level, the transponder determines the field strength of the first test signal received and sends a response signal with information about said field strength, the base station stores the received field strength information strength in a memory device, the transponder sends a second test signal with a predetermined power level, the base station determines the field strength of the second test signal received, compares the received field strength with the stored field strength and, in the event of any discrepancy which is greater than a predetermined level, determines that the transponder is located outside the near area of the transmitting antenna of the base station. 
 
   
   
     6. A method for identifying a portable transponder as being located in the near area of a transmitting antenna of a base station, comprising the steps of:
 transmitting a request signal to the portable transponder; 
 determining the field strength of the transmitted signal in the transponder; 
 transmitting a response signal to the base station; 
 transmitting the determined field strength to the base station; 
 determining the field strength of the response signal; 
 comparing the field strengths of the request signal and the response signal. 
 
   
   
     7. The method of  claim 6 , wherein the transmission of the response signal and the determined field strength of the request signal are combined to a single transmission. 
   
   
     8. The method of  claim 6 , wherein the determined field strength is transmitted together with a first response signal to the base station and wherein a second response signal is transmitted after a time delay. 
   
   
     9. The method of  claim 6 , wherein the base station stores the determined field strength of the request signal. 
   
   
     10. The method of  claim 6 , wherein the base station determines that the transponder is within the near area if the difference between the field strength of the request signal and the response signal are within a predefined range. 
   
   
     11. The method of  claim 6 , wherein the request signal comprises an identification number. 
   
   
     12. The method of  claim 6 , wherein the transmitted signals are data encrypted. 
   
   
     13. The method of  claim 7 , wherein the combined signals are data encrypted.